US 7593437 B2
Laser apparatus including two different, pulsed MOPAs, one having a fundamental wavelength of 1064 nm and the other having a fundamental wavelength of 1547 nm, provide trains of optical pulses. The 1064-nm pulses are frequency-quadrupled to a wavelength of 266 nm. The 1547-nm pulses are first mixed with the 266-nm pulses to provide pulses having a wavelength of 227 nm. The 227-nm pulses are then mixed with residual 1547-nm pulses from the first mixing to provide 198-nm output pulses of the apparatus.
1. A method of generating optical pulses, comprising the steps of:
generating pulsed radiation having a first fundamental wavelength from a first laser, said first fundamental wavelength being between about 975 nm and 1150 nm;
generating pulsed radiation having a second fundamental wavelength from a second laser, said second fundamental wavelength being between about 1500 nm and 1599 nm;
frequency-multiplying said first-wavelength radiation to provide radiation having a wavelength which is a harmonic-wavelength of said first fundamental wavelength, said frequency multiplying step being performed using at least two, serially arranged non-linear crystals;
in a first sum-frequency mixing step, sum-frequency mixing said harmonic-wavelength radiation with said second-fundamental-wavelength radiation to provide radiation having a first frequency-converted wavelength that is less than said harmonic-wavelength; and
in a second sum-frequency mixing step, sum-frequency mixing said first frequency-converted-wavelength radiation with non-frequency multiplied, unconverted, second-fundamental-wavelength radiation to provide frequency-converted radiation having a second frequency-converted wavelength, said second frequency-converted wavelength being less than said first frequency-converted wavelength and being less than 200 nm.
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9. A method of generating optical pulses, comprising the steps of:
generating pulsed radiation having a first fundamental wavelength from a first laser, said first fundamental wavelength being between about 975 nm and 1150 nm;
generating pulsed radiation having a second fundamental wavelength from a second laser said second fundamental wavelength being between about 1500 nm and 1599 nm;
frequency-multiplying said first-wavelength output radiation to provide radiation having a wavelength which is the fourth harmonic-wavelength of said first fundamental wavelength, said frequency multiplying step being performed using at least two, serially arranged non-linear crystals;
in a first sum-frequency mixing step, sum-frequency mixing said fourth-harmonic-wavelength radiation with said second-fundamental-wavelength radiation to provide radiation having a first frequency-converted wavelength that is less than said harmonic-wavelength, and residual second-fundamental-wavelength radiation; and
in a second sum-frequency step, sum-frequency mixing said first frequency-converted-wavelength radiation with said residual portion of non-frequency multiplied, unconverted, second-fundamental-wavelength radiation from said first sum-frequency mixing step to provide frequency-converted radiation having a second frequency-converted wavelength, said second frequency-converted wavelength being less than about 200 nm.
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13. Apparatus for generating pulses of optical radiation, comprising:
a first laser apparatus arranged to generate pulses of radiation having a first fundamental wavelength between about 975 nm and 1150 nm;
first and second optically nonlinear crystals arranged to generate pulses having the fourth-harmonic wavelength of said first fundamental wavelength, from said first-fundamental-wavelength pulses;
a second laser apparatus arranged to generate pulses of radiation having a first fundamental wavelength between about 1500 nm and 1599 nm;
a third optically nonlinear crystal arranged to sum-frequency mix said fourth-harmonic radiation pulses with said second-fundamental-wavelength pulses to provide pulses having a first frequency-converted wavelength and residual second-fundamental-wavelength pulses, said first frequency-converted wavelength being less than said fourth-harmonic wavelength;
a fourth optically nonlinear crystal arranged to sum-frequency mix said first-frequency-converted-wavelength pulses with said residual non-frequency multiplied, unconverted, second-fundamental-wavelength pulses to provide pulses having a second frequency-converted wavelength, said second frequency-converted wavelength being less than said frequency-converted wavelength and less than 200 nm.
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The present invention relates in general to fiber lasers. The invention relates in particular to generating ultraviolet (UV) radiation by frequency-converting the fundamental-wavelength output of fiber lasers having a fundamental wavelength of 1000 nanometers (nm) or longer, i.e., a wavelength in the near infrared (NIR).
UV laser radiation at wavelengths less than 200 nm and average power greater than 1 Watt (W) is useful in industrial applications such as laser machining, lithography, and optical inspection. Presently, the only laser types that will generate 1 W or more of such radiation directly, i.e., as the fundamental wavelength, are excimer and molecular fluorine (F) lasers. These lasers are very expensive to operate and maintain compared with other laser types such as diode-pumped solid-state (DPSS) lasers, including fiber lasers, which, unfortunately, have fundamental wavelengths at 900 nm or more.
Generation of sub-200 nm UV radiation from a DPSS laser having a fundamental wavelength greater than 900 nm requires that the fundamental output of the laser be frequency converted by frequency-doubling and sum-frequency mixing in a series of optically nonlinear crystals. In order to convert the output of such lasers having a wavelength of 1000 nm or more to a wavelength less than 200 nm, conversion would have to be to the sixth or higher harmonic. Harmonic conversion is limited, however, by the availability of optically nonlinear crystal materials that can transmit UV radiation less than 200 nm. A crystal of cesium lithium borate (CLBO) is presently the most preferred crystal for converting at wavelengths less than 200 nm, but even so, is limited to converting to wavelengths longer than about 190 nm.
Ytterbium-doped (Yb-doped) fiber lasers and neodymium-doped (Nd-doped) yttrium aluminum garnet (YAG) lasers have a fundamental wavelength of about 1064 nm. The sixth harmonic of this fundamental wavelength is about 177 nm, which is shorter than can be converted in CLBO. The fifth harmonic however is a wavelength longer than 200 nm n. Erbium-doped (Er-doped) fiber-lasers can generate fundamental radiation at wavelengths between about 1510 nm and 1590 nm. The eighth harmonic (8H) of any of these wavelengths longer than 1520 nm would be less than 200 nm and within the conversion range of CLBO.
Schemes for generating the eighth harmonic of the output of an Er-doped fiber laser are disclosed in U.S. Pat. No. 6,590,698. In one conversion scheme disclosed therein, the second harmonic (2H) is generated in a first optically nonlinear crystal. The third-harmonic (3H) is generated in a second optically nonlinear crystal by sum frequency mixing the 2H-radiation with residual fundamental radiation. Fourth-harmonic (4H) radiation is generated by frequency doubling 2H-radiation in a third optically nonlinear crystal. A fourth optically nonlinear crystal sum-frequency mixes the 3H- and 4H-radiation to generate seventh-harmonic (7H) radiation having a wavelength of about 220 nm, and a fifth optically nonlinear crystal generates 8H-radiation (about 193-nm radiation) by sum-frequency mixing the 7H radiation with residual fundamental radiation.
As any sum-frequency mixing or frequency-doubling operation in an optically nonlinear crystal is at best about 80% efficient, but typically is only about 50% efficient, the overall conversion efficiency from a cascade of five such operations will be less than 3%. This would require a laser having a fundamental power of 32 W in order to provide UV (less than 200 nm) radiation having a power of more than 1 W.
U.S. patent application Ser. No. 11/387,400, filed Mar. 23, 2006, assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated by reference, discloses a UV generation apparatus in which UV output is generated by sum frequency mixing harmonics of each of two lasers, and fundamental radiation from one of the two lasers. Calculations indicate that an average output power of about 1.0 W of 198 m radiation can be generated from about 8.7 W of 1064-nm fundamental radiation and about 7.1 Watts of 1564 nm fundamental radiation in five frequency-conversion stages. This is a total fundamental power of about 15.8 W, and represents a total-fundamental-power to 198-nm-output conversion efficiency of about 6.6%. It is not clear from the disclosure whether or not there would be a significant improvement in efficiency if higher fundamental power were available. However, it can be concluded from the disclosure that in order to effect a significant increase of the output power it would be necessary to increase the fundamental output power of each of the two lasers about equally. This could add significant cost to the apparatus.
There is a need for a continuing increase in efficiency and output power in apparatus for generating sub-200 nm wavelength radiation by frequency multiplication of the output of solid-state lasers. Preferably this should be achieved without a percentage increase in cost of the apparatus that is less than the percentage increase in output power.
The present invention is directed to methods of generating UV optical pulses, in particular, to generating optical pulses having a wavelength less than 200 nm. In one aspect, the invention comprises delivering radiation having a first fundamental wavelength from a first laser, and delivering radiation having a second fundamental wavelength from a second laser. The first fundamental wavelength is between about 975 nm and 1100 nm, and the second fundamental wavelength is between about 1500 nm and 1599 nm. The first-fundamental-wavelength radiation is frequency converted to provide radiation having a wavelength which is a harmonic-wavelength of the first fundamental wavelength. The harmonic-wavelength radiation is sum frequency mixed with the second-fundamental-wavelength radiation to provide radiation having a first frequency-converted wavelength that is less than the harmonic-wavelength. The first frequency-converted-wavelength radiation is sum frequency mixed with the second-fundamental-wavelength radiation to provide frequency-converted output radiation which has a second frequency-converted wavelength, the second frequency-converted wavelength being less than the first frequency-converted wavelength.
In examples of the inventive method, the first laser provides repetitive pulsed output at a wavelength of 1064 nm, and the second laser provides repetitive pulsed output at a wavelength of about 1547 mm. The 1064-nm pulses are frequency quadrupled in two frequency multiplication stages to provide pulses having a wavelength of 266 nm. In a third frequency conversion stage, the 266-nm pulses are sum frequency mixed with 1547-nm pulses from the second laser to provide pulses having a wavelength of 227 nm. In a fourth frequency conversion stage the 227-nm pulses are sum frequency mixed with residual 1547-nm pulses from the third frequency conversion stage to provide output pulses having a wavelength of 198 nm.
Calculations indicate that the 198-nm output pulses can have 1.0 W of average power for an average power output of about 13.3. W delivered from the first laser, and about 9.2 W delivered from the second laser. Calculations indicate that the 198-nm output pulses can have 10.0 W of average power for an average power output of about 39.2 W from the first laser and about 15.5 W from the second laser. This indicates that scaling output power in the inventive apparatus can be achieved primarily by scaling the output power of the first (shorter-wavelength) laser. The 10 W of 198-nm average output power is achieved at an efficiency of conversion of the total fundamental output power of the two lasers of about 18% (0.18).
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.
Referring now to the drawings, wherein like components are designated by like reference numerals,
Apparatus 20 includes generic fiber laser MOPAs 22 and 28. MOPA 22 includes a fiber master oscillator 24 (seed-laser) providing fundamental radiation at a wavelength between about 1000 and 1099 nm (designated in
The oscillator is preferably operated in a continuous-wave (CW) mode with the CW output being modulated, preferably by a modulator such as an integrated Mach-Zehnder (MZ) modulator. At the 10XX nm wavelength, it may be found advantageous to employ two such modulators in series to ensure an acceptable contrast ratio. Laser 24 can also be a fiber laser, distributed feedback (DFB) or distributed Bragg reflector (DBR) diode laser, an extended cavity diode laser (with a wavelength stabilizing fiber Bragg grating in close proximity to the diode), or a solid-state laser. For most of the above-mentioned lasers a wavelength locking mechanism is provided by an integrated grating structure in the cavity. If a precise control of a central wavelength is required then an external wavelength locker (38) can be used. In that case, a portion of the CW radiation is directed to a wavelength locker 38 that maintains a predetermined operating wavelength of the laser. Pulses output by the modulated fiber laser are amplified by a bulk (solid-state) amplifier 26. Laser 24 may also be provided with a fiber pre-amplification stage. This is discussed in detail further hereinbelow. As fiber lasers, fiber amplifiers, wavelength lockers and MZ modulators are well known in the art to which the present invention pertains, and a detailed description thereof is not necessary for understanding principles of the present invention, such a detailed description is not presented herein.
MOPA 28 is arranged similar to MOPA 22. A fiber laser 30 of MOPA 28 includes an Er-doped gain fiber. Laser 30 is operated in the same manner as laser 24 of MOPA 22 and, in this example, provides laser pulses having a wavelength between about 1500 nm and 1599 nm (designated in
Pulse delivery by MOPAs 22 and 28 is controlled by a controller 37 cooperative with a 5-MHz oscillator 34, a phase shifter 36, and the integral MZ modulators (not explicitly shown) of the master oscillators. A radio frequency (RF), here, 5 MHz, signal voltage from oscillator 34 is delivered to one electrode of the MZ modulator (or modulators) of master oscillator 24 and via phase shifter 36 to one electrode of the MZ modulator of master oscillator 30. Controller 37 provides digital signals to another electrode of the MZ modulators of the master oscillators for keying the MZ modulators. Each master oscillator delivers a train of pulses at a pulse repetition frequency (PRF) that is determined by the frequency of oscillator 34, and with a pulse duration that is determined by the keying signals applied to the MZ modulators. The phase difference between the two pulse trains is controlled by controller 37, in cooperation with phase shifter 34, using standard phase-shift-keying (PSK) techniques. MOPAs as described here will deliver pulses at a PRF in the megahertz range with pulse durations of less than 5 ns and even less than 1 ns.
It should be noted, here, that while the above described modulation scheme is a preferred modulation scheme, other modulation schemes may be employed without departing from the spirit and scope of the present invention. By way of example, master oscillators 24 and 30 may be directly modulated by modulating the optical pump source of the lasers. Whatever modulation scheme is employed, however, there must be some provision for adjusting the relative phase of pulse trains emitted by the lasers.
Provision of phase control is important in apparatus 20, as frequency-converted pulses from each MOPA are required to be further frequency converted by at least one optically nonlinear crystal, common to both. The fiber length in each MOPA amplifier will almost certainly be different. Beam paths followed delivery of pulses from each MOPA to a common crystal will also almost certainly be different. This being the case, and given that a 1-ns pulse has an optical path length in air of only about 30 centimeters (cm), phase control between the pulse trains generated by the MOPAs must be provided to ensure that the corresponding frequency converted pulses arrive simultaneously (temporally overlapping) at the common optically nonlinear crystal, thereby allowing further frequency conversion to take place. Phase control can be automatically implemented by detecting the mixing product output of any common optically nonlinear crystal, and communicating this output to controller 37. Controller 37 can then command phase shifter 36 to adjust the relative phase of the MOPAs until the detected mixing product is maximized. This phase control also enables a method of either digitally modulating or amplitude modulating UV output pulses of the apparatus.
Continuing with reference to
Amplified 15XX-nm pulses from fiber amplifier 32 follow a path B2. The 2XX-nm pulses from crystal 46 proceed along path B1 and are incident on a face 52A of an optically nonlinear crystal 52. Crystal 52 is preferably a CLBO crystal between about 10 mm and about 15 mm long, and is cut and arranged such that the 2XX-nm radiation is incident at Brewster's angle for the crystal material at that wavelength. Path B2 is folded by mirrors 54 and 56 such that 15XX-nm pulses traveling therealong are incident on face 52A of crystal 52 at an angle close to Brewster's angle for the crystal material at the 15XX-nm wavelength, such that the 15XX-nm radiation propagates substantially collinear with the 2XX-nm radiation within crystal 52. This means, for a CLBO crystal, that there will be an angle of about 1.6 degrees between paths B1 and B2 at face 52A of the crystal. Crystal 52, in this example is arranged for Type-I phase-matching for the 2XX-nm and 15XX-nm wavelengths and generates radiation pulses having a wavelength of about 2YY-nm (where 2YY is less than 2XX) by sum-frequency mixing, provided, of course, the above-described phase control between the MOPAs is adjusted such that the 2XX-nm and 15XX-nm radiation arrive simultaneously at crystal 52.
The 2YY-nm radiation pulses exit crystal 52 via face 52B thereof along a path B4. A beam sampler 72, for example, a tilted, uncoated calcium fluoride (CaF2) plate, directs a portion (for example, less than 1%) of the output of crystal 52 to a high speed UV photodiode 74. The output of photodiode 74 is transmitted to controller 37 for phase control implementation as discussed above. The remaining portion of the 2YY-nm pulses are incident on a face 58A of an optically nonlinear crystal 58. Crystal 58 is also preferably a CLBO crystal, about 15 mm long, and cut and arranged for Type-I phase matching for the 2YY-nm wavelength and residual 15XX radiation. Path B3, along which residual 15XX-nm radiation pulses are propagating, is folded by mirrors 60 and 62 such that the 15XX-nm radiation pulses are incident on face 58A of crystal 58 at an angle close to Brewster's angle for the crystal material at the 1064-nm wavelength, such that the 2YY-nm radiation propagates substantially collinear with the 15XX-nm radiation inside crystal 58. For a CLBO crystal, there will be an angle of about 4.5 degrees between paths B3 and B4 at face 58A of crystal 58. Crystal 58 generates 19X-nm radiation (output) pulses by sum-frequency mixing the 2YY-nm and residual 15XX-nm input pulses. Care must be taken to match the optical length of paths B3 and B4 between crystal 52 and crystal 58 such that the desired phase relationship of the 15XX-nm and 2YY-nm pulses is maintained at crystal 58. The 19X-nm output pulses exit crystal 58 via face 58B thereof along a beam path B5. Any residual (longer) wavelength pulses exiting crystal 58 will be propagating at some angle to path B5 and can be separated from the 198-nm pulses by spatial filtering.
It is important that output pulses from MOPA 22 have about the same temporal pulse width as output pulses from MOPA 28. This is because sum-frequency mixing can only occur when both radiations are co-propagating in the optically nonlinear crystals in which the mixing is taking place. In apparatus 20, the MZ modulator arrangement in MOPAs provides a means of accurately selecting and controlling temporal pulse widths.
It should be noted here that while CLBO is a particularly preferred crystal material for crystal 58, there is another crystal material, potassium aluminum borate (KABO) that may also be more or less useful, depending on the particular wavelengths that are to be finally mixed. The material has a phase-matching limit that extends to shorter fundamental wavelengths than that of CLBO, has a transparency comparable to CLBO and has a nonlinear coefficient that is between about 0.2 pM/V and 0.45 pM/V. This material, however, has not yet been commercially developed. Other possible crystal materials are potassium beryllium barium fluoride (KBBF), and yttrium aluminum borate (YAB), which also in the early stages of commercial development.
Those skilled in the art will recognize without further illustration that instead of using residual 15XX-nm radiation for the sum frequency mixing in crystal 58, it is possible to divide the 15XX radiation output of MOPA 28 two portions using a beamsplitter or the like, then use one portion for sum frequency mixing in crystal 52 and the other portion for sum frequency mixing in crystal 58. This, is not as efficient however as the sum frequency mixing arrangement using residual 15XX radiation described above with reference to
It should be noted here that a major shortcoming of 15xx-nm Er:Yb-doped fiber amplifiers is a low conversion efficiency (of pump power to output power), for example, between about 25% and 35%. By way of comparison Yb-doped fiber amplifiers for 10XX-nm amplification have a conversion efficiency between about 50% and 80%. Because of this, an increase of output power from an Er:Yb fiber amplifier by a factor of two will require at least between about 2 and 3 times more pump power than would be required to provide the same increase in a Yb-doped fiber amplifier. Further, existing bulk amplifiers at 1510-1590 nm, wherein gain media are typically Er:Yb glasses, have poor thermal properties and power scaling compared to those of bulk amplifiers for 10xx-nm, which typically employ crystal gain media. Accordingly, power up-scaling at 15xx-nm, while preserving a narrow linewidth of optical radiation, is more difficult and expensive than power up-scaling at 10XX nm.
Apparatus 20 has certain advantages over prior-art apparatus in that by employing two lasers, the power required to be produced by the 15XX-nm laser is reduced compared with above discussed schemes in which only an Er-doped fiber laser is employed. In the inventive scheme, each laser is operating at a wavelength close to a peak-gain wavelength. The total number of frequency conversion (sum-frequency mixing or harmonic generating) stages for the apparatus is only four. An advantage of the apparatus relating to the frequency conversion architecture thereof is that combining beam paths B1 and B2 and beam paths B3 and B4 by Brewster's angle incidence at the corresponding crystal faces eliminates a requirement for dichroic mirrors to provide such beam-path combination. At wavelengths less than about 400 nm, even the best commercially available such mirrors are lossy to some extent, and become increasingly lossy the shorter the wavelength. Such mirrors are also subject to degradation by short-wavelength UV radiation.
Dashed line EMAX in the graph of
Dashed line EMAX in the graph of
The efficiencies calculated by the graphs of
It is emphasized, here, that the present invention is not limited to using two pulsed lasers (or MOPAs) of any particular type. Preferably, however, any laser used as one of the two lasers in the inventive apparatus should provide a fundamental wavelength between about 800 nm and 1700 nm. Any two lasers used in the inventive apparatus preferably either inherently deliver, or can be controlled to deliver, pulses of about the same duration. Any two lasers used in the inventive apparatus must also be capable of being synchronized such that frequency multiplied (harmonic) pulses generated from the shorter-wavelength laser can be delivered simultaneously to an optically nonlinear crystal arranged to sum-frequency mix the harmonic pulses, with pulses of fundamental-wavelength radiation from the longer-wavelength laser.
It is emphasized again that the frequency-converted-output modulation scheme described above is not limited to use with the optical fiber MOPAs of
The output of each diode-laser 118 is coupled into cladding of the gain-fiber by a fiber 120 fused into the cladding of the gain fiber. An isolator 114 prevents feedback from amplifier stage 112 into the diode-laser. Amplified (pre-amplified) pulses are delivered from first amplifier stage 112 into a second fiber-amplifier stage 122, here, configured similarly to the first amplifier stage. Further pre-amplified pulses from amplifier stage 122 are delivered via an optical arrangement (not shown) to solid-state Nd:YVO4 amplifier 27 of laser apparatus 22A (see
Amplifier 136 is a double-pass amplifier including an erbium-doped gain-fiber 138 having a fiber Bragg grating (FBG) 140 at a distal end thereof and written into the core of the gain-fiber. FBG 140 is strongly reflective at a wavelength of 1547 nm and has a reflection bandwidth of about 1 nm or less. The distal end of the gain fiber is connected to a first port 143 of a wavelength division multiplexer (WDM) 142. Gain fiber 138 is optically pumped by CW radiation delivered by a diode-laser 144 and having a wavelength of 980 nm. The radiation from diode-laser 144 is coupled into gain-fiber 138 via a second port 146 of WDM 142. The FBG 140 reflects pulses amplified on a first pass through gain-fiber back through the gain fiber for amplification in a return pass. Most of any amplified spontaneous emission (ASE) generated in the first (forward) pass direction in the gain-fiber is transmitted by FBG 144, enters port 143 of the WDM, and exits the WDM via a third port 148 thereof.
Pulses that are pre-amplified in double-pass fiber amplifier 136 return to circulator 134 and are directed by the circulator into a second optical fiber amplifier stage 150 for further pre-amplification. Amplifier stage 150 includes an ytterbium-sensitized erbium-doped gain-fiber 152, optically pumped by a plurality (here, two) of diode-lasers 154, emitting CW radiation at a wavelength of 980 nm. The output of each diode-laser 154 is coupled into cladding of the gain-fiber by a fiber 156 fused into the cladding of the gain fiber. Amplified pulses from amplifier stage 150 are delivered via an optical arrangement (not shown) to Er:Yb:LMA fiber amplifier 32 as discussed above. Lasers (MOPAs) amplified by Er:Yb:LMA fiber amplifiers are capable of providing an average power output of up to 15 W for 1.0 ns pulses delivered at 5.0 MHz. This, as can be seen from the graphs of
In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.